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→‎The QT interval
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[[Image:QRSinterval.jpg|thumb|300px| The QT interval start at the onset of the Q wave and ends where the tangent line for the steepest part of the T wave intersects with the baseline of the ECG. Click on the image for a larger image]]
[[Image:QRSinterval.jpg|thumb|300px| The QT interval start at the onset of the Q wave and ends where the tangent line for the steepest part of the T wave intersects with the baseline of the ECG. Click on the image for a larger image]]
'''The normal QTc interval'''
'''The normal QTc interval'''
The QT interval indicates how fast the ventricles are repolarized and how fast they are ready for a new heart cycle.  
The QT interval indicates how fast the ventricles are repolarized and how fast they are ready for a new heart cycle.
The normal value for QTc(orrected) is: '''below 450ms for men and below 460ms for women'''. <cite>aha2</cite>
The normal value for QTc(orrected) is: '''below 450ms for men and below 460ms for women'''. <cite>aha2</cite>
If QTc is < 340ms [[Short_QT_Syndrome|short QT syndrome]] can be considered.


The QT interval comprises the QRS-complex, the ST-segment, and the T-wave. One difficultly of QT interpretation is that the QT interval gets shorter if the heart rate increases. This can be solved by correcting the QT time for heart rate using the Bazett formula: [[Image:Formule_QTc.png]]
The QT interval comprises the QRS-complex, the ST-segment, and the T-wave. One difficultly of QT interpretation is that the QT interval gets shorter if the heart rate increases. This can be solved by correcting the QT time for heart rate using the Bazett formula: [[Image:Formule_QTc.png]]
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Thus at a heart rate of 60 bpm, the RR interval is 1 second and the QTc equals QT/1. The [[QTc calculator]] can be used to easily calculate QTc from the QT and the heart rate or RR interval.
Thus at a heart rate of 60 bpm, the RR interval is 1 second and the QTc equals QT/1. The [[QTc calculator]] can be used to easily calculate QTc from the QT and the heart rate or RR interval.


On modern ECG machines, the QTc is given. However, the machines are not always capable of recognizing the correct QT time. Therefore, it is important to check this manually.
On modern ECG machines, the QTc is given. However, the machines are not always capable in the correct determination of the end of the T wave. Therefore, it is important to check the QT time manually.
'''Risk of QT prolongation'''
A prolonged QT interval can lead to arrhythmias such as [[Torsade de Pointes]] and subsequent [[Ventricular Fibrillation]].
{| class="wikitable"
! Causes of QT prolongation
|-
|
The QT interval is prolonged in congenital [[long QT syndrome]], but QT prolongation can also occur as a consequence of (a.o.):
* medication (anti-arrhythmics, tricyclic antidepressants, phenothiazedes, for a complete list look on [http://www.torsades.org Torsades.org]
* electrolyte imbalances
* ischemia.
QT prolongation is often treated with beta blockers.
|}


 
'''Correct measurement of the QT interval''': how to measure the end of the T wave
=====Correct measurement of the QT interval: how to measure the end of the T wave====
[[Image:Surawicz.jpg|120px|thumb|right|Dr Surawicz, still active in November 2006, at age 89.]]
[[Image:Surawicz.jpg|120px|thumb|left|Dr Surawicz, still active in November 2006, at age 89.]]
Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is poor as different definitions of the end of the T wave exist.<cite>Viskin</cite> Most QT experts define the end of the T wave as the intersection of the steepest tangent line from the end of the T-wave with the base line of the ECG.<cite>Lepeschkin</cite> This leads to the following stepwise approach:
Although QT prolongation is potentially lethal, measurement of the QT interval by physicians is poor as different definitions of the end of the T wave exist.<cite>Viskin</cite> Most QT experts define the end of the T wave as the intersection of the steepest tangent line from the end of the T-wave with the base line of the ECG.<cite>Lepeschkin</cite> This leads to the following stepwise approach:


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In  a (serious) prolonged QT time, it takes longer for the myocardial cells to be ready for a new cardiac cycle. There is a possibility that some cells are not yet repolarized, but that a new cardiac cycle is already initiated. These cells are at risk for uncontrolled depolarization and induce a [[Arrhythmias#Torsade_de_pointes|torsade de pointes]], a ventricular tachycardia.
In  a (serious) prolonged QT time, it takes longer for the myocardial cells to be ready for a new cardiac cycle. There is a possibility that some cells are not yet repolarized, but that a new cardiac cycle is already initiated. These cells are at risk for uncontrolled depolarization and induce [[Torsade de Pointes]] and subsequent [[Ventricular Fibrillation]].
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===How to measure QT if the QT segement is abnormal===
{| class="wikitable"
! Causes of QT prolongation
|-
|
The QT interval is prolonged in congenital [[long QT syndrome]], but QT prolongation can also occur as a consequence of (a.o.):
* medication (anti-arrhythmics, tricyclic antidepressants, phenothiazedes, for a complete list look on [http://www.torsades.org Torsades.org]
* electrolyte imbalances
* ischemia.
QT prolongation is often treated with beta blockers.
|}
 
If the QT segment is abnormal, it can be difficult to define the end of the T wave. Below are a number of examples that suggest how QT should be measured in these patients.
{| class="wikitable" width="750px" style="margin:5px"
{| class="wikitable" width="750px" style="margin:5px"
!colspan='3'|How to measure QT if the QT segement is abnormal
!colspan='3'|How to measure QT if the QT segement is abnormal
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===Short QT syndrome===
If QTc is < 340ms [[Short_QT_Syndrome|short QT syndrome]] can be considered.
{{clr}}
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== References ==
== References ==
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